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OCT & Optic nervePr. Jean-Philippe NordmannGlaucoma Centre - Hôpital des Quinze-Vingts, Paris

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Pr. Jean-Philippe Nordmann

Centre Hospitalier National d’Ophtalmologie des Quinze-Vingts 28, rue de Charenton75012 Paris - France

[email protected]

Preface

Like many complementary examinations in ophthalmology, OCT was designed as a collaboration between ophthalmologists and orthoptists. This book is also based on such a collaboration. I want to thank the orthoptics team of Quinze-Vingts Hospital, especially Mrs. Frédérique Brion, Audrey Payeras, Cybelle Blatrix and Mr. Meddy Metref. Thanks to their energy, most diseases of the optic nerve have been presented here and OCTs have been compared with other complementary examinations, in particular visual fields and photographs of the optic nerve. I would also like to thank Prof. Philippe Denis, President of the French Society of Ophthalmology for his help and in particular the provision of interesting examples.

Knowledge has no meaning unless shared. Laboratoires Théa and Carl Zeiss have agreed to write, publish and distribute this book free of charge.I am very grateful to them.

Introduction Page 13

PrinciplesofOCT Page 16

General principles Page 18

What does OCT really show? Page 20

Analysis methods of ocular structures with OCT Page 22

- Optic nerve Page 22

- Optical fibres around the papilla Page 23

- Complex of macular ganglion cells Page 24

Error sources with OCT Page 26

-Signal receiving errors Page 26

-Errors related to the positioning of the head Page 26

- Errors related to the optics of the eye Page 29

- Errors related to ghost images Page 30

PresentationoftheresultsinOCT Page 32

Cirrus™ HD-OCT (Carl Zeiss) Page 34

- Presentation “head of optic nerve and retinal nerve fibre analysis”

(RNFL and ONH: Optic Disc Cube) Page 34

- Presentation “Analysis of macular ganglion cells: Macular Cube” Page 37

OCT Optovue (RTVue) Page 38

- Presentation “Layer of parapapillary retinal nerve fibres and optic nerve” Page 38

- General presentation “Macular ganglion cells and layer

of retinal parapapillary nerve fibres” Page 39

Glaucoma Page 40

The three structures to be analysed by OCT in glaucoma Page 42

- Optic nerve Page 42

- Retinal nerve fibres (RNFL) Page 42

- Complex of macular ganglion cells Page 44

- Optic nerve or retinal nerve fibres: that look first in glaucoma? Page 45

- What if there is an isolated optic nerve damage? Page 46

Preperimetric glaucoma Page 48

- Preperimetric glaucoma: impairment isolated from the OCT Page 48

- Preperimetric glaucoma: impairment of the RNFL layer and the FDT Matrix visual field Page 50

- Preperimetric glaucoma: impairment of the RNFL layer, macula

and the FDT Matrix visual field Page 52

- Preperimetric glaucoma: impairment of the FDT Matrix, OCT at the limit of normal Page 54

Contents

Early open-angle glaucoma Page 56

- Glaucoma with incipient open angle: location of the impairment matches

between OCT and the visual field Page 56

- Early open-angle glaucoma: depth of the impairment matches

between OCT and the visual field Page 58

- Early open-angle glaucoma: primary impairment in the OCT Page 60

- Early open-angle glaucoma: primary impairment of the optic nerve

without impairment in the optical fibres Page 62

- Early open-angle glaucoma: comparison between OCT “Time Domain”

and “Spectral Domain” Page 64

Moderate open-angle glaucoma moderate Page 66

- Glaucoma with moderate open angle: impairment matches in the OCT and the visual field Page 66

- Moderate open-angle glaucoma: primary disease in the OCT Page 68

Advanced open-angle glaucoma Page 70

- Glaucoma with advanced open angle: impairment matches in the OCT and the visual field Page 70

- Advanced open-angle glaucoma: primary impairment in the visual field Page 72

- Advanced open-angle glaucoma: impairment does not match between OCT and the visual field Page 74

Normal pressure glaucoma Page 76

- Normal-pressure glaucoma: isolated fascicular deficit Page 76

- Early normal pressure glaucoma Page 78

- Moderate normal pressure glaucoma Page 80

Angle-closure glaucoma or sequelae of acute hypertension Page 82

Follow-up of open-angle glaucoma Page 84

- Can OCT improve in glaucoma? Page 87

- Comparison between “Spectral Domain” OCTs in glaucoma Page 87

- OCT and analysis of the cribriform lamina Page 87

Non-glaucomatousopticneuropathies Page 88

Multiple sclerosis 1

- Acute neuropathy and MS Page 91

- Correlation between OCT and visual field in MS Page 92

- Location of impairment by OCT in MS Page 93

- Major sequela of neuropathy in MS Page 94

- Can we detect subclinical MS neuropathy through OCT? Page 96

Acute optic neuropathy unrelated to MS Page 98

Toxic optic neuropathy Page 100

- Toxic optic neuropathy without apparent papillary impairment Page 100

- Toxic optic neuropathy with papillary and macular impairment Page 102

Anterior ischemic neuropathy Page 104

Optic neuropathy and uveo-papillitis 6

Papillary oedema Page 107

- Isolated oedema of the papilla without retinal diffusion Page 108

- Oedema of the optic disc with subretinal oedema Page 110

Compression of the optic nerve Page 112

Chiasma impairment Page 114

Amblyopia Page 116

Perinatal impairment of the central nervous system Page 118

Impairment of the central nervous system in adults Page 120

Determination of the organic character of visual impairment Page 122

Non ophthalmological neurodegenerative pathologies Page 124

AtypicalappearancesoftheopticnervethatmayshowopticneuropathyinOCT Page 126

Myopia Page 128

Dysversion the optic disc Page 130

Physiological excavation Page 132

Papillary coloboma Page 134

AtypicalappearancesoftheretinathatmayshowopticneuropathyinOCT Page 136

Vein occlusion sequelae Page 138

Pigmentary retinopathy Page 140

Conclusion Page 143

References Page 145

13

Introduction

OCT (Optical Coherence Tomography for) is a relatively new technique that allows images of biological tissue to be obtained by measuring the reflection of light from the structure in question. Depending on the wavelength used, the discernible details are in the range of 1 to 15 µm, i.e. at least twice as thin as is possible with the highest-performance conventional methods such as MRI or high-resolution ultrasound echography. Its main limitation is the need to analyse structures allowing sufficient light to pass through to obtain a reflected image.

The eye is clearly an “ideal” organ in this sense, because a very large number of ocular structures are mainly or partially transparent: cornea, lens, vitreous humour, neurosensory retina, as well as the front layers of the iris. Other highly reflective surface structures such as the pigment epithelium of the retina can be studied.

In 10 years, OCT has developed a key role in the diagnosis and follow-up of retinal and particularly macular impairments. This is due to the very descriptive nature of OCT images that seem to reproduce an almost histological appear-ance of the lesions observed. The latest generations of OCT, by multiplying the measurements, can not only reproduce this descriptive aspect, but can also perform a quantitative analysis of structures: thickness of the retina and each of its components such as the ganglion cell layer, analysis of the neuro-retinal rim, etc.

Retinal imaging by OCT has thus emerged as a crucial element for the analysis of glaucomatous disease, since it is now possible to quantify the thickness of the nerve fibre layer (called either optical fibre layer or retinal nerve fibre layer, RNFL), a structure that is predominantly affected by this impairment. The optic nerve has also benefited from this quantification (surface of the neuroretinal rim, cavity volume, etc.), so that all parts of the eye that are potentially altered in glaucoma have become accessible with OCT. Obtaining quantitative values and comparing them with the standards allows us to determine the existence and severity of glaucomatous disease and allows for follow- up.

It is also possible to analyse the iridocorneal angle, but in a less precise way, especially because what happens at the back of the pigment epithelium of the iris remains inaccessible to OCT.

Glaucoma is one of a range of optic neuropathies, and OCT also appears useful in the analysis of all diseases of the optic nerve and, to some extent, the central nervous system when ocular impacts are present.

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However, as with any new technology, analysis of the results of OCT requires careful interpretation because the method of data acquisition is complex and does not involve simple photography of the structures studied. It is always desirable to examine the quality of the results, not only in an overview, but also in closer detail when a zone appears suspect.

Once these pitfalls have been avoided, a new field of exploration and in particular investigations opens up. What is the clinical meaning of one impair-ment or another? How should a worsening of the impairment of the optical fibres be interpreted if all other exams are stable? How can we explain, on the contrary, that a particular structure is no longer changing in the OCT when the glaucoma is indisputably worsening?

OCT is advancing so quickly that it is impossible to clearly define its use pro-file, and any document is certain to become obsolete quite quickly. Currently indispensable for analysing the retina, it is becoming so for glaucoma and other optic neuropathies. This little book aims to simply give an update on OCT in glaucoma and pathologies of the optic nerve. The first chapter concerns the principles and interpretation of OCT and has a purely practical function, to help the reader understand the results. Glaucoma is obviously an important part of this book, but one part is devoted to other eye diseases and especially neuro-ophthalmological diseases, because they can mimic glaucoma or be a source of confusion if they are also present. Regardless of this facet of differential diagnosis, OCT has acquired its place in the evaluation of the optic nerve in general.

Introduction

Cirrus™ HD-OCT (Carl Zeiss)

OCT works by analysing the light reflected by zones crossed by an incident light generated by a laser with a wavelength in the infrared range around 840 nm. Logically, when a beam of light passes through a structure, one part of the light will continue its path (especially if the structure is quite transparent), one part will beabsorbed by the structure, one part will be reflected in all directions and the final part will be reflected towards the emission zone.

It is this last part of the light that is analysed in OCT. It corresponds to between one-billionth and one-millionth of the incident light, so it is very weak.

PrinciplesofOCT

16 17

18

Sound waves propagate relatively slowly (300m/sec in air) and it is possible to record the return time of this wave directly, as with UBM. However, the high propagation speed of light waves (300,000 km/sec) does not allow this time to be recorded, which is in the order of 30 femtoseconds (30 x 10-15 sec).

OCT uses the principle of interferometry to analyse this delay. An incident wave is divided into two, one part being projected onto a plane mirror and the other onto the eye. The two waves thus created are reflected; the wave sent to the mirror returns as a single echo; the wave sent to the eye returns as multiple echoes depending on the structure it has passed through. These waves are compared by an interferometer which measures the coherence between them (hence the term OCT, Optical Coherence Tomography). This allows us to deduce the thickness of the structure passed through by the wave sent to the eye.

This simple measurement at a given point and a given depth is called an A-scan. With “Time Domain” technology, the mirror moves repeatedly at dif-ferent depths to analyse the different layers of the retina.

Generalprinciples

Acquisition of an A-scan: Time Domain

“Time Domain” OCT technology: a reference mirror moves to successively study the different depths of the retina.

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Principles of OCT

The fastest devices in Time Domain allow the measurement of 17,000 A-scans per second. These measurements in one dimension are then reassembled into 2 dimensions to obtain a slice of the retina at a given place (B-scan). Time Domain technology is limited by these movement of the mirrors, since an examination cannot reasonably take more than a few seconds when it is necessary for the eye to remain relatively fixed.

A very different technology that has led to a dramatic improvement in the quality of OCT images is “Spectral Domain” technology. Instead of using the coherence between two waves, it uses the interference spectrum between the two reflected beams of broad-spectrum waves. This interference is studied by mathematical analysis using the Fourier transform. The great advantage of this “Spectral/Fourier Domain” technology is that it is possible to analyse the reflected ray not successively at each depth, but at the same time. The reference mirror therefore no longer has to move and this allows the process to become 50 to 100 times faster.

It is thus possible to make much more precise measurements (around 2 µm) during a reasonable examination time. Nevertheless, the examination itself is not instantaneous like a single photograph, and the quality of the results may be affected by an eye movement. It is not instantaneous because if the different depths of the retina can be studied at the same time, it is necessary for the laser beam to successively scan the different regions of this retina.

Acquisition of an A-scan: Spectral Domain

“Spectral Domain” OCT technology: the reference mirror is fixed and the different depths of the retina are analysed at the same time.

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OCT shows the reflection of light in different eye structures. This reflection is particularly strong when there are sharp edges between two media having different refractive indices. This is particularly the case in the cornea, which is perfectly imaged in OCT. In addition, these reflections are sharper when the structure crossed is perpendicular to the incident light, rather a glass that partially reflects light. In general, reflection of light (only a part of which occurs towards the incident source) is a property of inhomogeneous structures responsible for microchanges in the refractive index, such as cell membranes, nuclei, cytoplasm, axons of cells, etc. The most reflective structures of the retina are the retinal nerve fibre layer (RNFL or optical fibres), the pigment epithelium and the interplexiform layers. The more reflective a structure is, the more red it appears, due to the colour code used in OCT.

Some structures contain melanin, which absorbs light strongly. At this level, reflection and absorption are responsible for an exponential reduction in the power of the incident beam, preventing it from exploring more distant areas. That is why OCT does not allow the precise study of structures beyond the pigment epithelium of the retina, which has the dual property of being highly reflective in the part first hit by light, and highly absorbent in the most basal area.

From this light reflection data, OCT measures the thickness of the layer studied in a calculation that considers the time that the incident beam takes to return and the known refractive index of the structure crossed.

It is therefore very important to note that OCT images are not direct images of the retina, but rather a reconstruction based on mathematical calculations transposed subsequently into images of the ocular fundus. You can understand this quite easily by looking for example at the changes in the appearance of the optical fibres in the optic nerve when these fibres change direction to exit the eye. The reflection of light on the layer of the optical fibres appears in red. When these fibres change direction when moving towards the optic nerve, this colour changes. OCT is therefore not an anatomical slice of the retina.

WhatdoesOCTreallyshow?

21

Principles of OCT

These simple remark allow us, in case of doubt about a result, to view the images obtained and get an idea about whether the impairment exists or not.

When OCT analyses a layer of cells (for example, the ganglion cell layer of the macula), it must be remembered that the measurement corresponds to the entire layer (ganglion cells, support cells, interstitial fluid, etc.) This explains why, when glaucoma is very advanced or even passed, the layer in question does not completely disappear, because some support cells remain.

Weak reflection because the layer of optical fibres becomes parallel to the light.

Strong reflection because the layer of optical fibres is perpen-dicular to the light.

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In this brief chapter we review only those structures liable to be modified in glaucoma and optic nerve pathologies, namely the optic nerve head, the parapapillary optical fibre layer and the macular ganglion cell complex.

Optic nerve

The initial problem of analysis of the optic nerve is how to define the con-tours of the optic nerve and what can be considered the start of the cavity. It was chosen to define a reference plane arbitrarily set at 150 µm above the level of the peripapillary pigment epithelium. Based on this level, everything below it is considered to correspond to an cavity, whether physiological or not. It is then possible to measure the papillary parameters: width and surface of the neuroretinal rim on the different meridians, size of the disc and cav-ity, C/D ratio, etc. It is important to understand these points because when the disc is irregular, the measurements may be distorted by erroneous identification of this plane.

Méthodesd’analysedesstructuresoculairesenOCT

Eye with normal configuration

The papillary region is regular, the pink line clearly determines the boundary between the retina and the cavity.

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Principles of OCT

Optical fibres around the optic disc

The device’s program first identifies the two most refractive zones of the retina, which are the front of the optical fibre layer and the pigment epithelium. It then evaluates the location where the reflection of the optical fibre layer decreases sharply, thus allowing its thickness to be measured. The speed of the “Spec-tral Domain” OCT examination allows the measuring of a cube around the optic nerve, but it is necessary to choose what zone is to be detailed. In fact, the thickness of the optical fibres is measured at 3.4 mm from the centre of the optic disc. This distance was chosen because it is the best compromise “Thickness of optical fibre layer/interindividual variability”. In fact, the closer you get to the disc, the thicker the layer is, and we could therefore be expected to be able to detect small changes in it. However, close to the op-tic disc, the results vary from one person to the next, and depending on the location of the vessels and the shape of the disc. Conversely, if the measurement is done a long way from the disc, the results are more consistent between people, but the fibre layer narrows quickly and small changes become less detectable. In practice, it was therefore decided to measure the fibre layer at 3.4 mm from the centre of the disc.

Strong myopia

The papillary region is irregular; the pink line does not readily demarcate the boundary between the retina and the cavity.

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However, it is not always at this level where the first disease signs are found or where follow-up will be done best. In the follow-up, the zones closest to the disc would be likely to allow a better analysis of progression.

Complex of macular ganglion cells

Thanks to “Spectral Domain” technology, it is possible to measure the thickness of each layer of the retina and in particular the ganglion cell layer in the macular region.

This zone is particularly interesting because the cells in this region correspond to around 30% of the entire thickness of the structure. The foveal region is not of interest because, conversely, it is endowed with too few ganglion cells.

Most devices measure a range including “internal interplexiform layer - bodies of ganglion cells - axons of ganglion cells passing above them”.

AnalysismethodsofocularstructureswithOCT

Circle located 3.4 mm from the centre of the optic nerve corre-sponding to the measuring zone of the optical fibre layer.

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Principles of OCT

It is possible to refine this measurement to better locate the impairment by eliminating the axons of ganglion cells that correspond to cell bodies located at a distance. This is offered by some devices (e.g. the Cirrus™ HD-OCT (Carl Zeiss)), which eliminates the axon layer and thus measures only the “internal interplexiform - ganglion cell bodies” layer.

The three layers studied in the complex of ganglion cells.

Retinal nerve fibresGanglion cells

Internal interplexiform

Diagram of the retina

ganglion cell

Light

layer of optical fibres

layer of ganglion cells internal

plexiform layer

external plexiform layer

internal granular layer

external granular layer

layer of external segments of the photoreceptors

pigment epithelium

rod

cone

horizontal cellamacrine

cell

bipolar cell

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OCT appears as an objective examination, so it is less prone to patient-related errors. However, many errors can occur, related either to the imaging or to the structure of the eye.

Signal receiving errors

The quality of imaging is crucial and is not straightforward, since only less than one-millionth of the incident light sent by the laser is reflected towards the sensor. This quality is expressed in the results by the expression “Signal Strength” and must be greater than 6/10 with the Cirrus™ HD-OCT (Carl Zeiss) or 50 with the Optovue (RTVue). A poor signal is often responsible for an underestimation of the thickness of the optical fibres.

Errors related to head positioning

The measuring principle of OCT is based on reflection of incident light, which depends on two key things: microchanges in the refractive index of the medium crossed, and the orientation of the structure crossed; the more the latter is perpendicular to the incident beam, the stronger the reflection. Head positioning errors lead to changes in the axis of the eye and are therefore responsible for variations in the results.

This can easily be seen in the example below, remembering that the colour code in OCT indicates that the redder the colour is, the more reflective it is. If the presentation is well done, the parapapillary optical fibre layer is thick.

When the depth setting is incorrect (for example, too distant), this layer seems to decrease strongly, or when the head is tilted back.

This does not always result in insufficient signal quality, and therefore it cannot be detected easily.

SourcesoferrorwithOCT

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Principles of OCT

Correct head position. The optical fibre layer (red) is normal.

Head tilted back. The optical fibre layer is reduced.Same eye as in the previous patient.

Poor adjustment in myopia. The optical fibre layer is reduced. Same eye as in the previous patient.

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Another factor responsible for poor quality is the artefact related to eye movements because the examination time is around 1.5 sec. OCT offers programs to correct these artefacts, or even eye tracking during the examination. Nevertheless, it should be ensured that there are no discontinuities in the results.

With OCT in the “Spectral Domain”, artefact phenomena in the mirror may occur when the eye is incorrectly positioned or when significant depth variations of the retina are present in severe myopia. The upper limit of the OCT images corresponds to immediate reflection (no delay in reflection) and therefore nothing can be weaker. If the eye is not positioned correctly, an even closer image will be detected in the mirror and therefore completely inverted, with the outer part of the eye appearing at the top and the inner part at the bottom. In addition to the optic nerve, the retinal layers are therefore also completely inverted.


Shifting of the image Discontinuity of the image during an examination due to an eye movement. This is responsible for errors in the calculation of the RNFL parameters.

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Principles of OCT

Errors related to the optics of the eye

The optics of the eye may also impact the result quality. Opacification of the posterior capsule tends to reduce the evaluation of the optical fibre layer 1. Conversely, wearing multifocal lenses does not change the results at both the macula and parapapillary levels 2. The size of the optic disc does not influ-ence the thickness of the optical fibres, except in the case of very small size where false disease signs seem to exist 3. Conversely, myopia is a source of artificial reduction of this thickness due to measurement problems when the axial length of the eye is too high. The “bigger” an eye is, the bigger the circumference located 3.4 mm from the centre of the optic nerve. The opti-cal fibres are more spread out than in a normal eye, and this therefore leads to a reduction in thickness, without however the number of optical fibres being fewer than normal 4. This phenomenon is not significant in the macular ganglion cells and we can therefore prefer to analyse this region in high myopic patients 5.

Patient incorrectly positioned for the right eye. The image touches the upper part of the black frames and all of the retinal structures appear inverted. The left eye is correct.

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Errors related to ghost images

OCT can sometimes confuse highly reflective structures that lie close to each other. This is the case, for example, in the posterior hyaloid and the internal boundary of the retina.

Layer of peripapillary optical fibres that is abnormally high due to vitreous traction in this region (visible as a blue line).


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In most diseases of the optic nerve, a thinning of the optical fibre layer is observed. However, the OCT images should be consulted in case of doubt, because this thinning may be hidden by oedema in the structure having a different origin. This is the case when traction phenomena are noted.

Bilateral glaucoma patient with impairment of visual field touch-ing the central 10°. On the right, we note an impairment of the ganglion cell complex. On the left, there is a macular hole. Tractions at this level and the oedema that follows mask the thinning of this layer, which seems to be normal.

Principles of OCT

Many different devices currently exist. In the United States, more than 30 devices have obtained a marketing authorisation. It is therefore not possible to give a detailed presentation of all available OCTs. In France, the most common devices are the Stratus and Cirrus™ HD-OCT (Carl Zeiss)and the Optovue (RTVue). Most of the examples provided were produced with these two devices. Insofar as the two most important areas in glaucoma are the papillary and parapapillary regions, on the one hand, and the macular ganglion cell complex on the other hand, these results are subject to detailed interpretation.

PresentationofresultsinOCT

32 33

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Presentation “head of optic nerve and retinal nerve fibre analysis” (RNFL and ONH: Optic Disc Cube)

The examination shows, on a single page, both eyes (OU), with the right eye (OD) located on the left and the left eye (OS) on the right. In all of the statistical presentations, the colour yellow indicates an anomaly at p < 5% and red indicates an anomaly at p < 1%. Some patterns are small and difficult to analyse, but it is possible to enlarge them on a separate page.

The values obtained are compared to a patient of the same age with an optic disc of the same size. The size limits of the optic disc range from 1.3 mm2 to 2.5 mm2. Outside of these limits, all measured parameters are shown in grey, because they are not compared to a standard. For each value, this number appears on a white background if it corresponds to 5% of the best values, and on a green background for 90% of cases. The colour yellow indicates an anomaly at p <5% and the colour red indicates an anomaly at p < 1%.

Cirrus™HD-OCT(CarlZeiss)

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Signal strength. This lets you know if the results have been correctly acquired. Values higher than 6/10 are required. Even if the signal is strong, the results may be inaccurate if the eye is incorrectly positioned.

Vertical section of the posterior pole. Imaged elements identical to the previous presentation.

Position of the retina at 3.4 mm from the optical centre, corre-sponding to the region where the retinal nerve fibres are studied.

Image of the ocular fundus in which the statistical values of anomalies in the optical fibre layer are coloured. The optic disc is delimited, as well as the zone 3.4 mm from the centre of the disc where the thickness of the parapapillary nerve fibres is measured. It is important that this circle is not located in a zone of peripapillary atrophy because the reflectance of the pigment epithelium would be evaluated incorrectly and the results could be distorted.

Horizontal section of the poste-rior pole. It is important to ensure that the retina does not touch the upper part of the square, as this would distort the results. The reflectance of the different layers appear, as well as the boundaries of the retina and Bruch’s mem-brane. Two small dots indicate a zone 150 µm above Bruch’s mem-brane that defines the start of the cavity.

Thickness of the optical fibre layer. The thicker this layer is, the warmer the colour. Given the physiology of optical fibres, a “watch hands” appearance centred on the disc is normal.

Full name of the patient. It must be spelled correctly in order to guarantee follow-up. The patient’s date of birth is impor-tant because the results are com-pared to age-dependent norms.

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Cirrus™HD-OCT(CarlZeiss)

Average thickness of the optical fibre layer (RNFL).Symmetry of this layer between the 2 eyes (RNFL Symmetry).Surface of the neuroretinal rim (ANR area).Area of the optical disc.Average C/D ratio.Average vertical C/D ratio.Cavity volume.

Thickness of the optical fibre layer in different regions (temporal, superior, nasal, inferior) at 3.4 mm from the centre of the optic nerve; the right eye is shown as a continuous line, the left eye as a dotted line, normal zones in green, abnormal zones in yellow and then red.

Representation for each eye of the thickness of the optical fibres, by sections 6 hr wide. In glaucoma, the most affected zones are the upper and lower zones.

Thickness of the neuroretinal rim in the different regions (tem-poral, superior, nasal, inferior, TSNIT); the right eye is shown as a continuous line, the left eye as a dotted line, the normal zones in green, abnormal zones in yellow and then red. This makes it easy to locate a notch.

Representations for each eye of the thickness of the optical fibres, by sections 2 hr wide. In glau-coma, the most affected areas are located at 7 hr for the right eye and 5 hr for the left eye.

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Presentation “Analysis of ganglion cells: Macular Cube” (Ganglion Cell OU Analysis)

The examination shows, on a single page, both eyes (OU), with the right eye (OD) located on the left and the left eye (OS) on the right. For each value, this number appears on a white background if it corresponds to 5% of the best values, and on a green background for 90% of cases. The colour yellow indicates an anomaly at p < 5% and the colour red indicates an anomaly at p < 1%.

Signal strength. This lets you know if the results have been correctly acquired. Values higher than 6/10 are required.

Horizontal scan of the macula passing through the fovea.

Presentation in the sector of the macular region, without studying the fovea, with the average thick-ness value in each zone.

Table of average and minimum values of the thickness of the layer “ganglion cells + internal interplexiform”, without the layer of ganglion cell fibres.The minimum value corresponds to the thinnest sector of 1° width.

Statistical values of anomalies in this layer. The colour yellow indicates an anomaly at p < 5% and the colour red indicates an anomaly at p < 1%.

Thickness of the layer of the ganglion cell complex. The thicker this layer is, the warmer the colour.

Full name of the patient. It must be spelled correctly in order to guarantee follow-up.

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Presentation “Layer of parapapillary retinal nerve fibres and optic nerve”

The examination shows the overall results for the head of the optic nerve and the parapapillary region, for one eye, with statistical analysis.

OCTOptovue(RTVue)

Head of the optic nerve

Thickness of the optical fibre layer in different regions (temporal, superior, lower nasal TSNIT) at 3.4 mm from the centre of the optic nerve.

Image of the ocular fundus; the circle indicates the zone where the optical fibre layer is mea-sured.

Full name of the patient. It must be spelled correctly in order to guarantee follow-up.

SSI: Signal Strength Intensity.

Parameters of the optic nerve head.

Analysis region of the optical fi-bres at 3.4 mm from the optical centre.

Thickness of the optical fibre layer in the different regions. The warmer the colour, the greater the thickness.

Thickness by region of the optical fibre layer with statistical values. Normal zones are green, abnor-mal zones are yellow and then red.

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General presentation “Macular ganglion cells and layer of parapapillary retinal nerve fibres“

A single page shows all of the results without adding a direct OCT image. Many elements described in the previous results are repeated here.

OCT Optovue (RTVue)

Parameters of the optic nerve head.

Thickness by region of the optical fibre layer with statistical values. Normal zones are green, abnor-mal zones are yellow and then red.

Interocular differences, layer of parapapillary fibres.

Complex of macular ganglion cells “layers of optical fibre axons + ganglion cells + internal interplexiform”. Statistical repre-sentation.

Thickness of the optical fibre layer in the different regions. The warmer the colour, the greater the thickness.

Glaucoma is a slowly progressive optic neuropathy that is most often associated with ocular hypertension. The progressive deformation of the head of the optic nerve caused by this hypertension leads to a cavity and destruction of the retinal nerve fibres passing through the cribriform lamina. This destruction is responsible for visual field impairment. This succession of “structural impairment inducing functional impairment” allows us to estimate that, in theory at least, the analysis of the structure should detect an initial impairment before the impairment of the visual field6. For a long time, the methods available to study the structure did not allow this approach to be confirmed.

Glaucoma

40 41

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With the new-generation “Spectral Domain” OCT, the structural changes can often be detected before they have impaired the visual field. However, in some cases, we note that the visual field is impaired before we can detect any structural impairment. Each eye has between 800,000 and 1.2 million optical fibres. If the “starting point“ is close to the upper limit, significant impairment can occur while the subject remains within the statistical standards. On the other hand, if the initial structure of the eye is more normal, destruction of fibres by glaucoma is more quickly visible.

Optic nerve

At the head of optic nerve, the parameters affected firstly in OCT are, in order: • the vertical thickness of the neuroretinal rim (not shown in the results), • the overall surface of this rim (Area of the ANR), • the vertical C/D ratio.Although these parameters detect glaucoma well, they are not very effective in differentiating early glaucoma from moderate glaucoma 7.

Retinal nerve fibres (RNFL)

The parameters that best differentiate normal subjects and early glaucoma subjects are, in order: • the thickness of the retinal nerve fibres in the lower temporal zone, • the thickness of the retinal nerve fibres in the lower quadrant, • the average thickness of retinal nerve fibres.

Some studies seem to show that the upper temporal sector would be just as differentiating as the lower temporal quadrant 8.

ThethreestructurestobeanalysedbyOCTinglaucoma

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The three structures to be analysed by OCT in glaucoma

Retinal nerve fibres- Average RNFL thickness.

Retinal nerve fibres- RNFL thickness in the lower

quadrant.

Retinal nerve fibres- RNFL thickness in the lower

temporal zone.

Optic nerve- Vertical thickness of the

neuroretinal rim,- Overall surface of the

neuroretinal rim.

- Vertical C/D ratio.

Main criteria of a glaucomatous disease in OCT

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First elements changed in glau-coma- Lower temporal thickness

First elements changed in glau-coma- Average minimum thickness

Complex of macular ganglion cells

The study of macular ganglion cells is more recent because it is only available in “Spectral Domain” OCT. The parameters most suggestive of early glaucoma are the average minimum thickness and the lower temporal thickness.

The macula is divided into 360 sectors each one degree wide. The average minimum thickness is that of the thinnest sector (Minimum GCL thickness).

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Optic nerve or retinal nerve fibres: which should be exam-ined first in glaucoma?

The information provided by OCT is diverse and raises the question of what parameters are most reliable for early detection of glaucoma and, in particular, whether it is better to analyse the optic nerve head or the para-papillary ganglion fibres. In fact, we can imagine that, when there is a large number of optical fibres, it is easy to analyse them and, when this thickness tends to reduce, the posterior edge of this layer is more difficult to differenti-ate from adjacent structures and therefore gives more variable measurements. By contrast, it would be easier to determine the parameters of the optic nerve, since only the only the vitreous body is facing these fibres and the determina-tion of the optic nerve head by analysis of the end of Bruch’s membrane is performed well by OCT.

However, the appearance of the optic nerve itself varies greatly from one person to another, and statistical comparisons are difficult. This is particularly the case when the optic nerve is irregular (myopia, dysversion, etc.).

In general, it is easier to detect preperimetric glaucoma in the retinal nerve fibres (RNFL). More rarely, analysis of the optic nerve is preferred. In some patients, analysis of the macular ganglion fibres would be as good as analysis of the RNFL layer 9.

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What about a case of isolated impairment of the optic nerve?

Generally, the analysis of the optic nerve in OCT is more subject to discussion than the analysis of optical fibres due to the diversity of appearances of the optic nerve from one person to another; this diversity is not perfectly covered by the standards databases of the devices. The result is that we sometimes find an isolated optic nerve impairment when the retinal nerve fibres appear normal, both at the parapapillary level and at the level of the macular ganglion cells when the visual field is also normal. In this situation, we must favour the analysis of optical fibres. However, it is desirable to analyse the progression of the parameters of the optic nerve since the comparison is then done on the patient himself.

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Patient with an appearance suggesting a physiological cavity. The entire results are normal, apart from an abnormal cavity volume for each eye. Glaucoma is not established. Simple moni-toring should be proposed.

Preperimetricglaucoma

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60-year-old patient with hyper-tension at 23 mmHg. The FDT Matrix and OCT of the optic nerve are at the limit of normal.

In general, the term “preperimetric glaucoma” is used for a glaucoma that is detected by an impairment of the structure or by an alteration of the early detection tests in the visual field, such as the blue-yellow visual field or the FDT Matrix, while the conventional visual field, i.e. the Humphrey-type auto-mated perimetry or Octopus is normal.

Preperimetric glaucoma: isolated impairment in OCT

Preperimetric glaucoma

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Same patient as on the previous page. The macular OCT shows a clear and well systematised impairment in the right eye. The 10° Matrix test is irregular.

The OCT can be the only modified examination in preperimetric glaucoma. In general, in this situation, simple monitoring is sufficient, but treatment is a viable alternative, especially if the deficit in the OCT is large.

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Preperimetric glaucoma: impairment of the RNFL layer and the FDT Matrix visual field

The start of impairment of the visual field by FDT Matrix and OCT corresponds to the beginning of glaucoma, especially if the deficits are consistent. These deficits may precede the appearance of a conventional perimetry deficit by 5 years.

74-year-old patient, ocular hyper-tension at 24 mmHg. Normal optic nerve. The OCT and FDT Matrix show matching impairment of the right eye.

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Same patient as on the previous page. There is no central impair-ment.


Preperimetric glaucoma: impairment of the RNFL layer, the macula and the FDT Matrix visual field

There is often an impairment of both the RNFL layer and the macular ganglion cell complex in OCT. If the deficit is more present in the parapapillary region, we can assume that hypertension likely plays a major role. Otherwise, we will consider more vascular problems.

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55-year-old patient with ocular hypertension at 26 mmHg. The automated perimetry is normal, but the Matrix shows a bilateral alteration. The OCT confirms the impairment, both in the optic nerve and the RNFL layer.


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Same patient as the previous page. There is little macular impairment at the right and a normal result on the left. This profile seems to suggest early hypertension glaucoma.

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Preperimetric glaucoma: impairment of the FDT Matrix, OCT at the limit of normal

It is rare to note a normal OCT in a patient who already has perimetric impair-ment. However, such cases can hypothetically occur in glaucoma. However, it should then suggest a more central impairment and lead to the performance of other tests with less doubt.

60-year-old patient with ocular hypertension at 24 mmHg. The Matrix visual field shows a marked impairment on the left, while the OCT is only slightly impaired.

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Same patient as on the previous page. OCT and central 10° visual fields. The macular ganglion cell complex is normal on the right and at the limit of normal on the left, while the Matrix 10° central test is pathological.

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Earlyopen-angleglaucoma

Early open-angle glaucoma: location of the matching impairment between the OCT and the visual field

The OCT is often consistent with the visual field in early and moderate glaucomas, except in young patients where the OCT impairment is greater than the impairment of the visual field.

79-year-old patient with ocular tonus at 22mmHg at the right and 26 mmHg at the left. At the right, the visual field is normal and the OCT shows a lower temporal deficit typical of early glaucoma. On the left, the impairment is more marked and extensive.

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Early open-angle glaucoma

In OCT, a more significant impairment in the macular region than the peripapil-lary region is not a severity factor if the deficit remains moderate.

Same patient as on the previous page. OCT and central visual fields of 10°. Impairment of the macular region of the left eye is observed. The average and minimum thick-ness parameters are abnormal. The central visual fields of 10° are normal.

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Early open-angle glaucoma: depth of the impairment matches between the OCT and the visual field

The impact on the visual field of a reduction in the thickness of the fibres depends on the severity of the impairment. It is therefore necessary to analyse the thickness precisely in the different sectors.

70-year-old patient affected by open-angle glaucoma since the age of 8. The impairment is almost concordant between the visual field and the OCT, with more marked changes on the left both at the level of the retinal nerve fibre and the level of the cavity.

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This may explain why an impairment may not be noticeable in the visual field in the central region if the most deficient point remains relatively normal. For this reason, the minimum thickness is presented in the macular OCT, in addition to the average thickness.

Same patient as on the previous page. OCT and central visual fields of 10°. Significant macular impair-ment of both sides, without nota-ble impact on the right visual field. This is due to the fact that, in the corresponding zone, the macular ganglion complex retains a high thickness of more than 60 µm. On the left, the thickness in the upper sector is reduced to 44 µm and causes a deep lower paracentral scotoma.

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31-year-old patient affected by open-angle glaucoma since the age of 4. Significant and almost isolated impairment of the OCT. All parameters for the optic nerve are abnormal, as well as most of the elements regarding the retinal nerve fibre layer.

Early open-angle glaucoma : OCT predominant impairment

The younger the patient, the more the OCT is disrupted before the visual field shows any changes. This is due to the redundancy of receptor fields in the ganglion cells. For each region of the visual field, several ganglion cells are present and process the same zone. This phenomenon decreases with age.

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Same patient as on the previous page. Significant macular impair-ment without loss of visual acuity or central visual field.

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68-year-old patient with ocular hypertension.

On the right, the diagnosis of glaucoma is in no doubt, both at the level of the RNFL layer and in the macular region.

On the left, we note that the reti-nal nerve fibre layer (RNFL) is not affected but the OCT parameters of the optical nerve are abnormal. The glaucoma is indeed bilateral. The slightly abnormal visual fields support the diagnosis.

Early open-angle glaucoma: primary impairment of the optic nerve without impairment of the optical fibres

The absence of impairment of the RNFL layer does not allow glaucoma to be ruled out. If there is a cavity noted clinically or in the OCT and it is is associated with a small impairment of the visual field, you should hold back on diagnosing glaucoma proper.

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Same patient. OCT and central visual fields of 10°. Bilateral impairment of the macular gan-glion cell layer. The central visual field is normal.

When the RNFL layer is normal, the analysis of the macular ganglion cells allows us to differentiated isolated hypertension and early impairment of optical fibres.

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Glaucoma. OCT Time Domain

Glaucoma with incipient open angle: comparison between OCT “Time Domain” and “Spectral Domain”

Comparison of OCT Time Domain (Stratus OCT, Carl Zeiss) and Spectral Domain (Cirrus™ HD-OCT, Carl Zeiss) shows that these two methods have the same detection capacity as regards incipient glaucoma. The main difference resides in the fact that Spectral Domain OCT is more reproducible and there-fore allows better monitoring than Time Domain 10.

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OCT « Time Domain » et « Spectral Domain »

The results obtained through Time Domain and Spectral Domain OCT are not identical. In severe glaucoma in particular, we note a 10 to 20% variation in the results. The “colour codes” of Stratus OCT and Cirrus™ HD-OCT are not interchangeable. In general, the thickness of RNFL is smaller with Cirrus than with Stratus.

Same patient as on the previous page. Spectral Domain OCT. The results are approximately compa-rable.

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Glaucomawithmoderateopenangle

Glaucoma with moderate open angle: concordant impairment with OCT and visual field testing

Perfect concordance between OCT and visual field testing is rare, as OCT often allows detection of slightly more serious impairment than perimetry testing.

Moderate glaucoma on the right. Visual field testing highlights a higher deficit. The optical fibres are only affected in the lower concordant region.

Right eye

Left eye

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Glaucoma with moderate open angle

Same patient as on the previous page. The analysis of macular ganglion cells confirms this im-pairment, which is concordant with visual field testing, but also shows a subclinical impairment in the upper region and in the other eye.

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Glaucomawithmoderateopenangle

Glaucoma with moderate open angle: OCT predominant impairment

OCT impairment in moderate glaucoma is sometimes more important than visual field testing can predict. Thus, OCT appears to be very useful not only for confirming perimetric deficit but also for analysing other regions appearing normal in visual field testing.

Moderate glaucoma. Visual field testing highlights an essentially higher deficit. The optical fibres are affected in the lower concor-dant region, but also the upper region. The right eye is normal in Matrix visual field testing and OCT.

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Glaucoma with moderate open angle

Same patient as on the previous page. OCT and central 10° visual fields. The analysis of macu-lar ganglion cells confirms that impairment is concordant with visual field testing, but also shows subclinical impairment in the upper region. The right eye is entirely normal.

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Glaucomawithadvancedopenangle

61-year-old patient with predomi-nant glaucoma, left. The OCT profile preserves a preferential impairment in the upper and lower sectors with relative preservation of the temporal fasciculus.

In advanced glaucoma, OCT confirms the impairment but visual field test-ing keeps its predominant position. In fact, other than certain seriousness, the thickness of optical fibre layers or that of macular ganglion cells is not reduced because of the presence, despite optical atrophy, of support struc-tures accounting for the residual thickness of the RNFL.

Glaucoma with advanced open angle: impairment concordant with OCT and visual field testing

In advanced glaucoma, a thickness of about 50 µm corresponds to the absolute deficit regions of visual field testing.

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Glaucoma with advanced open angle

Same patient as on the previous page. OCT and central 10° visual fields. Moderate impairment present, right. All macular regions are affected, left. Note that a simple difference of 15 µm to 20 µm separates incipient deficit (right eye) and absolute deficit zones (left eye).

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84-year-old patient with advanced glaucoma and reduced visual acuity, right. On this side, the visual field MD is -26dB. It is -11dB on the left. In OCT, this difference is less pronounced, the aver-age thickness of the RNFL being 54 µm on the right against 60 µm on the left.

Right eye

Left eye

Glaucoma with advanced open angle: predominant impairment of visual field testing

In the most advanced stages of glaucoma, it is difficult to rely on the thickness of the optical fibres, both at the papillary and the macular level. This thickness is only slightly reduced in this situation.

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Same patient as on the previous page. OCT and central 10° vi-sual fields. The same is noted as regards the absence of a clear correlation between VF deficit and thickness of the ganglion cell complex. The MD is -20dB right and -9dB left. The average thick-nesses of the macular fibres are 57 µm right and 58 µm left.

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65-year-old female patient with more or less symmetrical bilateral glaucoma. Nevertheless, the RNFL only appears to be really abnor-mal on the right. This could be due to measurement errors associated with serious sparkling synchysis of the left eye, well visualised in OCT and responsible for mediocre signal quality.

“Drop” aspect associated with vitreous opacities of the left eye.

Glaucoma with advanced open angle: discordant impairment between OCT and visual field testing

Sometimes, OCT only shows moderate impairment despite an obviously abnormal visual field. This could be due to measurement errors or real discordance.

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Same patient as on the previous page. OCT and central 10° visual fields. Macular OCT affected on both sides.

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Normalpressureglaucoma

60-year-old female patient, ocu-lar pressure of 15 mm Hg without treatment. The results on the right are normal. A slight fasciculate deficit is observed on the left, both in the optical nerve and the parapapillary fibres. The optical nerve parameters are normal.

In normal pressure glaucoma, we often observe central 10° visual field impair-ment. Due to the high number of ganglion cells in this central region, this is expressed by considerable reduction of the retina surface and, therefore, important excavation at the advanced stage. OCT also tends to find identical characteristics. Often, an isolated or predominant impairment of the macular region is found.

Normal-pressure glaucoma: isolated fascicular deficit

An isolated deficit can be found in normal-pressure glaucoma, expressed by a notch in the optical nerve head and the RNFL.

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Normal pressure glaucoma

Same patient as on the previ-ous page. OCT and central 10° visual fields. Very localised impairment of the left macular ganglion cell complex.

In OCT, the characteristics of normal-pressure glaucoma are, at the early stage, predominant or isolated impairment of the macular region.

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Incipient normal-pressure glaucoma

In the initial phases, average RNFL is less affected in normal-pressure glaucoma than in hypertonic glaucoma, which is logical because the affected fasciculi are finer 12.

68-year-old female patient, ocu-lar pressure of 16 mmHg without treatment. OCT shows localised impairment, right. Average RNFL thickness is not affected but the optical nerve parameters are affected, right.

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Same patient as on the previous page. OCT and central 10° visual fields. Using OCT, very localised lower macular impairment is observed, which is concordant with the perimetric deficit.

In GPN, the paracentral scotomas are deep and absolute, and only briefly pass through a relative scotoma phase. In contrast, OCT allows quantification of the optical fibre layer of the macular region, which can reduce gradually while no functional indication is present.

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60-year-old female patient affected by normal-pressure glaucoma. The right eye associ-ates the usual indications of open-angle glaucoma with a nasal jump and arched scotoma. The RNFL remains relatively undisturbed.

A small paracentral scotoma appears on the left, which only slightly alters the thickness of the optical fibre layer.

Right eye

Left eye

Moderate normal-pressure glaucoma

Using OCT, the distinction between normal-pressure glaucoma and high- pressure glaucoma is found, essentially, in the initial phases of the dis-ease. When glaucoma is more advanced and the visual filed is affected more specifically, this difference between normal-pressure glaucoma and hyper-tonic glaucoma disappears gradually.

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Same patient as on the previous page. OCT and central 10° visual fields. The central 10° test and OCT are concordant. Contrary to what one often observes in hyper-tonic glaucoma, not all the macu-lar sectors are damaged.

The analysis of the macular ganglion cell complex available in “Spectral Domain” OCT allows to detect the beginning of impairment, which is expressed by highly localised reduction of the fibres, with less global macular impair-ment than in GPAO.

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Closed-angleglaucomaorsequelaeofacutehypertension

58-year-old male patient who underwent a closed-angle glau-coma crisis in the left eye a year earlier, resolvent under medical treatment and iridectomy.Pressure is 16 mmHg on each side. Matrix and OCT normal on the right. On the left, the visual field appears normal but in OCT upper impairment is shown. The optical nerve parameters show slightly abnormal results on the left.

Immediately after a glaucoma crisis by reducing the angle, OCT does not change significantly. Nevertheless, 3 to 9 months later, the optical fibre layer is reduced both locally in the upper and lower regions and globally, while the perimeter remains unchanged in most cases. It might thus be interesting to compare OCT directly and some time after the acute crisis in order to establish the occurrence of impairment sequelae 13.

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Closed-angle glaucoma or sequelae of acute hypertension

Same patient. Impairment of the macular region of the left eye is observed. The minimum average thickness parameter is abnormal.

Even in the absence of an acute crisis and if pressure is normal, the thick-ness of the optical fibres in the low temporal region is sometimes smaller in patients with narrow angles, Asians in particular 14. This suggests repercus-sions of nocturnal pressure outbursts.

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Open-angleglaucomamonitoring

Open-angle glaucoma monitoring

OCT allows detection and monitoring of glaucoma evolution. “Spectral Domain” OCT being recent, it is difficult to find long-term longitudinal studies; the most recent being a 4-year monitoring of patients15. Several types of evolution are possible. The most frequent is an expansion of the deficit surface, with a tendency to approach the macula. Less frequently, one observes a deepening of the impairment or the appearance of impairment in a different region. In order to detect this aggravation, the zone selected usu-ally, 3.4 mm from the centre of the optical nerve, is too peripheral; a region at 2mm being more differentiating. In more than 50% of the cases, this aggrava-tion in OCT is not confirmed by automated perimetry testing, thus underlining the low correlation between these two examinations, at least during a short evolution period. On the contrary, in 20% of the cases, isolated aggravation in the perimeter is confirmed without OCT evolution.

Cirrus HD-OCT proposes a glaucoma monitoring programme and an analysis of event evolution (GPA Report).

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GPA monitoring parameters

Optical fibre thickness.

Changes in the thickness of the optical fibres between exami-nations, when a first change is detected, are drawn in yellow, if confirmed during the next exami-nation, they appear in red.

Changes, through time, of the average thickness of the RNFL globally, in the upper region and the lower region (3 tracings).

Average C/D ratio evolution.

Synthesis of the probability of stability or aggravation for three key parameters, fibre thickness and TSNIT (focal progression) and average thickness (diffused pro-gression), as well as average C/D.

Fibre thickness profile in the vari-ous regions (TSNIT), using the same colour code in the case of aggravation.

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Open-angleglaucomamonitoring

This GPA programme allows, like in perimetry testing, to detect significant evolution 16. Nevertheless, important limits persist because evolution is con-sidered real in patients with glaucoma if the new examination is outside the variability limits as determined in healthy individuals. We do not know whether this variability is smaller in reality or greater in patients with glaucoma. In the study by Leung et col 17, an increase in the volume of the optical fibre layer is even found in 13% of the cases, which is difficult to accept because that would signify an improvement of glaucoma. Undoubtedly, this result corresponds to initial measurement errors. The same team had carried out a similar study 3 years earlier using OCT Stratus. This study shows a minimal capacity to detect progression with previous generation OCT.

Aggravation of structural impairment and functional impairment do not evolve in parallel. These works thus clearly show that it is not possible to avoid auto-mated perimetry testing, when this examination is abnormal, in the monitoring of glaucoma, but that OCT also appears to be very useful, at least in the incipi-ent and moderate glaucoma phases.

When evaluating the aggravation of glaucoma through the reduction of optical fibre thickness, one should consider the glaucoma stage. In fact, the relation-ship between optical fibre loss and visual field aggravation is not linear 18. In the initial phases of glaucoma, an optical fibre loss can increase without an important change in the visual field. At a more advanced stage, a moderate change in optical fibre thickness is expressed by an important aggravation of the visual field. Finally, in the later stages, the optical fibre thickness does not change practically, while the visual field continues to worsen. The persis-tence of a certain optical fibre layer thickness does not mean that a significant number of gangion cells are present, but rather that cells supporting this layer are still present.

Simultaneous aggravation of visual field testing and OCT in 6 months. A 3 micron decrease of the average thickness of the RNFL is considered significant. Here, in the left eye, it changes from 78 µm to 74 µm.

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Can OCT improve in glaucoma?

In practice, OCT does not improve in glaucoma. Nevertheless, after an impor-tant reduction in ocular pressure through surgery, OCT can show an increase in the thickness of optical fibre layers, but this phenomenon is temporary and only lasts about 3 months 19. Most often, OCT improvement is the consequence of better images that were not optimal during the first measurements.

Comparison between “Spectral Domain” OCT in glaucoma

Except for 1st generation OCT (Time Domain), there are several Spectral Domain OCT devices: Cirrus™ HD-OCT (Carl Zeiss), 100RTVue (Optovue), Spectralis (Heidelberg)… These devices use the same operating principle but vary as regards acquisition speed, the existence of a system that monitors view and the retinal layers segmentation method.

It is not possible to use the gross values of Spectral Domain OCT and trans-pose them to another. The results with these devices are very close but different, e.g. Cirrus HD-OCT gives lower RNFL values than RTvue 20. In particu-lar, this is associated with the measurement zone, which differs slightly from one device to another.

OCT and cribriform plate analysis

OCT is in constant development. OCT allows analysing practically all structures that can be illuminated. Apart from optical fibre impairment, we know that glaucoma leads to deformation of the cribriform plate. Thanks to a specific programme (Enhanced Depth Imaging-Optical Coherence Tomography (EDI-OCT)), in glaucoma, one can visualise the cribriform plate deforming towards the back, but also shifting backwards in relation to the sclera in certain merid-ians 21. We do not know whether this corresponds to a cause or a consequence of impairment of this structure, but it appears that after surgical reduction of ocular pressure, the cribriform plate gains anterior adherence and thickens again 22. Thus, this analysis appears to be an important line of development not as regards the content (optical fibres) but the crossed structure (position of the cribriform plate, sectoral thinning, form of pores) 23. In contrast, there does not appear to be a great interest in measuring the thickness of the choroids, which is only slightly changed by glaucoma 24. Nevertheless, in case of peripapillary atrophy, this thickness is not reduced 25.

In essence, optic neuropathies are responsible for the deterioration of optical fibre axons and cause OCT modification. This impairment affects both the parapapillary and the macular fibres. OCT has thus become a very important diagnosis tool in neuro-ophthalmology 26. In many cases, the OCT profile helps diagnosis and evaluation of the importance of optic neuropathy. However, the OCT “profile” of certain diseases resembles that of glaucoma. It thus appears important to know the OCT semeiology of these different neuropathies.

Neuropathies-opticnonglaucomatous

88 89

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Multiplesclerosis

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Multiple sclerosis

In multiple sclerosis, the analysis of the retinal ganglion fibre layer is of great interest because it is better correlated with the functional symptomatology of the patients (lower acuity or reduced visual field) than in other supplementary examinations, such as MRI 27.

Acute neuropathy and MS

During the acute phase of MS optic neuropathy, the RNFL is sometimes more voluminous than normal. This provides evidence of slight papillary oedema, which is not clinically perceptible. This oedema is present even if the demy-elinating plaque is quite poeterior.

Thinning of the optical fibres layer occurs between the 1st and 3rd month after the acute crisis and stabilises around the 6th month. In the absence of a new crisis, there is no aggravation after 6 to 8 months.

The most damaged site is the macular area where about 34% of the retinal volume is composed of the ganglion cell layer. In this pathology, an eye affected by optic neuropathy presents a reduction of about 35 to 45% in macular optical fibre thickness i.e. 20 µm to 40 µm, for a normal thickness of 110 µm to 120 µm 28. The contralateral eye is also affected in most cases, but to a lesser degree (20% reduction in thickness). In order for impairment to appear in perimetry testing, a 75 µm reduction approximately is necessary.

It is thus very important to analyse OCT in relation with neuropathy dynamics

29. In the acute phase, the absence of layer thinning is falsely reassuring. In fact, impairment is delayed.

The papilla and macula are affected by MS. In the initial phases, the RNFL can be artificially enlarged by papillary oedema, which is not the case for macular ganglion cells. In the long-term, the two structures develop in parallel 30.

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Multiplesclerosis

Multiple sclerosis after an out-burst. The optic nerves are nor-mal. OCT shows predominant impairment in the temporal sec-tors of each eye. This deficit does not appear in Goldmann’s kinetic perimetry testing. Temporal im-pairment is typical of non glauco-matous neuropathies.

Right eye

Left eye

Correlation between OCT and visual field testing in MS

When OCT impairment is minimal, the visual field and visual acuity remain normal in general after an acute crisis. In contrast, if a residual thickness threshold of 75 µm is affected in the peripapillary area, the field is disturbed.

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Multiple sclerosis

Same patient as on the previous page. OCT and central 10° visual fields. Impairment of macular ganglion cells is major in each eye, despite a very slightly affected central visual field.This type of diffused impairment is found frequently.

Localisation of OCT impairment in MS

The temporal quadrant is the most frequent location. This allows distinguish-ing it from glaucoma where impairment is rather superior or inferior.

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Multiplesclerosis

Female patient affected by MS, with several optic neuropathy outbursts. The visual field of the left eye is highly affected. OCT confirms this impairment and also reveals a practically identical change in the right eye. The optic nerves show predominant bilat-eral optical atrophy on the left. There is no pathological excava-tion.

Right eye

Left eye

Important sequelae of MS neuropathy

When multiple sclerosis evolves, visual field and OCT deteriorate. This evolution is not always parallel, OCT being able to appear more affected than the visual field.

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Multiple sclerosis

Same patient as on the previous page. Impairment of ganglion cells is major on both sides.

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Multiplesclerosis

Female patient affected by MS without known optic neuropathy episodes. OCT shows irregular impairment in the various sectors, while the visual field is normal.

Right eye

Left eye

Is it possible to detect subclinical MS neuropathy using OCT?

OCT is very useful for analysing the contralateral eye of the eye affected by acute neuropathy and in the absence of ocular impairment in patients with known MS. In fact, it allows detection of subclinical impairment and also monitoring of the neurological state of patients, because impairment of the visual pathways is almost always present in this disease 31.

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Multiple sclerosis

Same patient as on the previous page. Diffused impairment visible. Normal visual acuity.

We could even perform OCT in order to monitor the evolution of treatments administered in general, in the absence of a visual history. This examination could thus provide a general evaluation of MS.

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AcuteopticneuropathynotassociatedwithMS.

30-year-old female patient with bilateral blurred vision, without reduced acuity. The MRI results do not indicate MS. The ocular fundus reveals more significant papillary oedema on the left than on the right. OCT confirms this result through a major increase in parapapillary optical fibre thickness on both sides. This result does not indicate that the optical fibres are healthy.

Right eye

Left eye

Ophthalmologists are often confronted with acute optic neuropathy of unknown origin, demyelinisating or not. Does OCT help to distinguish demy-elinisating optic neuropathy, whether lone or associated with other types of pathology? The clinical context is essential. In favour of demyelinisating aetiology: occurring between the age of 20 and 50; ocular pain during eye movement in particular; strictly unilateral character; progressive aggravation in about a week; start of recuperation after one month.

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Acute optic neuropathy not associated with MS.

Same patient as on the previous page. The analysis of ganglion cells shows important impairment on the left, while visual acuity is 10/10th. Observation of the macula, far from the papilla, allows precise monitoring of the repercussions of optic neuropathy.

Unfortunately, the OCT profile does not allow differentiation of such impair-ment, which is expressed by initial increase in fibre thickness in the case of oedema, and then further reduction of the RNFL.

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Toxicopticneuropathy

61-year-old male patient affected by optic neuropathy, undoubtedly associated with alcohol/nicotine. The optic nerves appear normal. The visual fields show bilateral centrocaecal deficit. OCT reveals papillary oedema. The RNFL ap-pears normal because it is artifi-cially increased due to oedema.

Right eye

Left eye

Toxic or nutritional optic neuropathies are suggested in the case of impair-ment, which is often bilateral, symmetrical and painless, classically associated with centrocaecal scotoma. OCT often reveals bilateral and gradual impair-ment of the RNFL 32. As regards the peripapillary optical fibres, we note a purely temporal deficit with relative preservation of the upper and lower sectors.

Apparent toxic optic neuropathy without papillary impairment

In the presence of associated papillary oedema, the aspect of the parapapil-lary fibres can appear normal because layer thickness is artificially increased by the oedema.

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Toxic optic neuropathy

Same patient as on the previous page. OCT and central 10° visual fields. The central 10° visual field confirms central impairment. Macular OCT shows significant bilateral, symmetrical and homo-geneous impairment.

In the presence of associated papillary oedema, we find a contrast between two normal parapapillary optical fibres and major impairment of the ganglion cell complex.

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42-year-old male patient affected by optic neuropathy, associated with alcohol/nicotine. The optic nerves are normal. Goldmann’s visual fields show distortion of the blind spot on the right and a normal result on the left. OCT reveals typical impairment af-ter bitemporal impairment. The upper and lower sectors are not affected. There is no pathological excavation.

Right eye

Left eye

Toxicopticneuropathy

Toxic optic neuropathy with papillary and macular impairment

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Same patient as on the previous page. OCT and central 10° visual fields. Slight diffused deficit of the central 10° visual field present on both sides. Macular OCT shows significant bilateral, symmetrical and homogeneous impairment. This aspect suggests toxic NORB.

Toxic optic neuropathy

In incipient toxic optic neuropathies, impairment can be more evident in the macular ganglion cells, even in the case of preserved visual acuity. In general, it is diffuse and symmetrical.

104

63-year-old female patient, hyper-metrope, affected 1 year earlier by anterior ischaemic neuropathy in the left eye. The visual field of the right eye is normal, while that of the left eye shows superior altitu-dinal deficit, which is characteris-tic of this pathology. OCT is coher-ent with this unilateral impairment and also shows secondary excava-tion on the left.

Right eye

Left eye

Anteriorischaemicneuropathy

It is sometimes difficult to distinguish remote sequelae of anterior ischaemic optic neuropathy (AION) and glaucoma using OCT. Impairment of the RNFL is temporal in both cases 33. The OCT deficit in AION is more altitudinal than in glaucoma, affecting not only the lower temporal sector (7hr for the right eye, 5hr for the left eye), but describing more extensive impairment 34. One can also expect that the excavation volume is smaller in AION 35.

105

Same patient as on the previous page. Macular OCT shows global impairment of the left eye. This aspect could not be more typical because one would expect a more altitudinal deficit here.

Anterior ischaemic neuropathy

During an acute phase of AION, initial papillary oedema causes spectacular increase of parapapillary optical fibre thickness, while the ganglion cell layer is not altered.

106

Left unilateral uveopapillitis, of toxoplasmic origin probably, in a 19-year-old woman. In the best case, OCT allows determination of the importance of papillary oedema and clarifies the impor-tance of associated subretinal oedema and its proximity in relation to the fovea.

Right eye

Left eye

Opticneuropathyanduveopapillitis

Inflammatory optic neuropathies within the framework of uveopapillitis ini-tially cause significant increase of the optical fibre layer, prior to secondary atrophy in half of the cases 36. OCT allows detection of the oedema and intra and subretinal localisation.

107

20-year-old male patient with intracranial hypertension due to an expansive intracranial process. OCT reveals an increase in volume of the optic nerve and RNFL thick-ness. Monitoring of the parame-ters of this layer allows measuring the evolution of this oedema.

Papillaoedema

Right eye

Left eye

Papillary oedema, whatever the cause may be, is responsible for spectacular increase of the optical fibre layer because OCT measures this layer in a non specific way. During regression of oedema, the RNFL will appear reduced, indicating the sequelae of the papillary oedema. The kinetics of the impair-ment is therefore essential. The speed of appearance of sequelar deficits depends on the aetiology of the oedema.

108

54-year-old female patient with intracranial hypertension

OCT reveals an increase in the vol-umes of the optical nerve without associated retinal impairment.

Papillaryoedema

Lone oedema of the papilla, without retinal diffusion

In cases of lone oedema, the optic nerve parameters describe an increase of the volume of the optic nerve, while the RNFL is normal. Nevertheless, a normal thickness can be the consequence of oedema associated with incipient optic atrophy. Long term assessment is necessary.

109

Same patient as on the previous page. Macular OCT is normal.

Papillary oedema

The absence of optic fibre impairment is rather a good prognosis in the case of macular oedema. A new assessment of the acute episode will allow confirming the stability of the results.

110

23-year-old female patient with discreet reduction of visual acuity of unexplained origin. The ocular fundus reveals bilateral papil-lary oedema and the visual fields reveal enlargement of the blind spot on both sides. OCT reveals an increase of RNFL thickness. The absence of optical fibre im-pairment is associated with this oedema.

Papillaryoedema

Right eye

Left eye

Papillary oedema with subretinal oedema

It is important to note that there is no associated subretinal oedema because, in this case, we can predict improvement of the visual function during regres-sion. If the oedema is not subretinal, recuperation will be weaker. OCT allows distinguishing intra and subretinal oedema in the parapapillary region easily.

111

Same patient as on the previous page. OCT and central 10° visual fields. In the macular region, the ganglion cell complex is already altered on the right. On the left, this impairment is hidden by oedema. We note the absence of good prognosis of subretinal oedema.

Papillary oedema

112

23-year-old female patient af-fected by glioma in the left optic nerve. Impairment is purely uni-lateral, underlining the absence of compression in the other eye. Although the optic nerve gives the impression of excavation, this is not confirmed by OCT.

Opticnervecompression

Right eye

Left eye

In the case of optic nerve compression, purely unilateral compression is present. The optic nerve is sometimes difficult to examine because optic atrophy can be accompanied by secondary excavation. Therefore, a glaucoma-tous hypothesis is sometimes mentioned.

113

Same patient as on the previous page. Diffused and lone macular impairment of the left eye is observed.

Optic nerve compression

In the case of optic nerve compression, the normality of the contralateral eye allows to define the purely localised repercussion of this compression.

114

Female patient affected by prechi-asmatic meningioma compressing the left optic nerve mainly. Acuity on the right is normal and on the left lower than 1/20th. The optic nerve is normal on the right and slightly pale on the left. OCT of the optic nerve head shows nasal and temporal impairment with very small fibre thickness (42 µm and 37 µm). There is no excavation.

While Goldmann’s visual field test-ing appears to show the begin-ning of impairment on the right, OCT remains normal.

Chiasmaimpairment

Right eye

Left eye

In the case of chiasma impairment, visual field and OCT lesions are present. After surgery, a favourable functional recuperation factor is the fact that, in the affected zone, the RNFL thickness of at least 80 µm 37 is maintained. Even if the thickness is smaller, in general, improvement is observed after the oper-ation. In compressive neuropathies, the volume of the optical fibre layer is thus an indicator of the probability of regression of the clinical indications after surgery. The greater the thickness of the fibres prior to treatment, the better recuperation will be.

115

Same patient as on the previous page. Diffused and lone macu-lar impairment of the left eye is observed. In the case of compres-sion, this diffused character is usually observed.

Chiasma impairment

The RNFL is not always affected at an early stage in optic nerve or chiasma compression, an alteration of the visual field sometimes being the first sign of impairment. This constitutes one of the rare cases where the absence of fibre impairment does not indicate a healthy situation.

116

48-year-old female female patient with amblyopia in the left eye (visual acuity 2/10th). Visual field testing shows nasal deficit of inexplicable origin. Peripapillary OCT is normal, as are the optic nerve parameters.

Amblyopia

Amblyopia is expressed by a lowering of visual acuity in the relevant eye. The ganglion cells are not functional from a morphoscopic point of view, but they are still present. OCT only appears slightly modified in this situation.

117

Same patient as on the previous page. OCT and central 10° visual fields. The central visual field of the left eye shows diffused deficit consistent with 2/10th acuity. OCT is normal.

Amblyopia

118

42-year-old female patient with sequelae of epilepsy and slight mental retardation associated with perinatal injury. The visual field is irregular with lower hom-onymous lateral deficit. The optic nerves are normal. Although this is central pathology, the OCT of the optical fibres is affected.

Perinatalimpairmentofthecentralnervoussystem

One would think that reduction of the optic fibre layer can only be found in the case of direct impairment of such cells, in the cell body or the axon. It appears that even a lone occipital cortex lesion can be expressed by hom-onymous lateral reduction of optical fibres in OCT. This was initially evidenced for congenital or perinatal pathologies. It is not certain whether this is the case for acquired lesions.

119

Same patient as on the previous page. OCT and central 10° visual fields. The central visual field is irregular and macular OCT shows bilateral impairment.

Perinatal impairment of the central nervous system

120

Impairmentofthecentralnervoussysteminadults

49-year-old female patient. The visual fields show inexplicable and relatively symmetrical bilat-eral deficit in the upper field. OCT is normal. The optic nerves also appear clinically normal.

Right eye

Left eye

Visual repercussions of lone impairment of the central nervous system in adults is expressed by much more important alteration of the visual field than OCT. Nevertheless, the OCT might be affected by transsynaptic degeneration from the occipital lobe to the optic nerve 38 .

121

Impairment of the central nervous system in adults

Same patient as on the previous page. OCT and central 10° visual fields.Minor impairment of the ganglion cell complex is in contrast with the importance of the perimetric deficit. A central origin remains the most probable aetiology.

OCT is thus very helpful when it is normal. With a lowering of visual acuity or impairment of the visual field, confirmation of the absence of retina destruc-turing in OCT allows confirming a neuro-ophthalmological cause for the symptoms.

122

Determinationoftheorganiccharacterofvisualimpairment

60-year-old male patient com-plaining about 6/10th reduction of visual acuity after general anaesthesia. The visual fields show inexplicable bilateral centro-caecal deficit. Neuropathy due to low choroidal debit is mentioned. OCT allows revealing major diffuse impairment of the RNFL, with-out excavation. The optic nerves appear clinically normal.

In many situations it is difficult to establish the organic origin of important functional symptoms. This is the case for post cranial trauma, perimetric defi-cit of hysteric origin or ill-defined pathologies.

OCT allows definition of the organic or non organic character of this functional complaint.

123

Determination of the organic character of visual impairment

Same patient as on the previous page. OCT and central 10° visual fields. The ganglion cell complex is impaired significantly, thus confirming the organic character of the patient’s complaint.

124

Nonophthalmologicalneurodegenerativepathologies

In more general pathologies such as Alzheimer’s disease or Parkinson, we also observe a reduction of optical fibres in OCT. In certain cases, this reduction is diffuse and sometimes located in the superior region 39.

In Alzheimer’s disease, a reduction of the RNFL and the macular ganglion cell layer is observed, but this is not directly related with the level of cognitive functions.

In Parkinson’s disease, a certain relationship between reduction of the optical fibre layer and the functional repercussions of Parkinson’s disease has been found.

Numerous atypical aspects of the optic nerve are responsible for the changes of papilla OCT. They can consider the default measurement methods and produce a wrong pathology result. OCT measures excavation in relation to a reference plan, arbitrarily set at 150 µm below the level of the peripapillary pigment epithelium. If OCT cannot evaluate correctly the pigment epithelium, all the results can be wrong. This is why, papilla sections, indicating with a red point the theoretical start of excavation, are printed. Ophthalmologists can thus determine whether the points are placed in a consistent way.

AtypicalaspectsoftheopticnervethatcansuggestopticneuropathyinOCT

126 127

128

Female patient with myopia of 6 dioptres and slight lower tempo-ral dysversion. Matrix visual field testing shows irregularities, OCT is normal.

Right eye

Left eye

Myopia

Moderate myopia does not cause OCT modification. In the case of peripapil-lary atrophy, it should be ensured that the measurement made at 3.4 mm from the centre of the optic nerve is located outside this atrophy, in order to obtain good quality measurements.

129

Same patient as on the previous page. Ganglion cell complex irreg-ularities on the right. This result only indicates that a pathological process is evolving.

Myopia

130

Female patient with bilateral dys-version. The visual field is affected on the right mainly, with alteration of the RNFL suggesting glaucoma. There is no excavation using OCT.

Right eye

Left eye

Papillarydysversion

Moderate dysversion of the papilla does not lead to OCT modifications, especially if it concerns the temporal axis. Nevertheless, in such cases we sometimes observe slight nasal deficit, an aspect that does not suggest glaucoma.

In contrast, if impairment is more important and especially if its axis is nasal, important alterations occur and can be sources of confusion.

The macular ganglion cell complex is simpler to evaluate in dysversion as it is located at a distance from the optic nerve. It allows revealing the existence of macular impairment.

131

Papillary dysversion

Same patient as on the previous page. OCT and central 10° visual fields. An analysis of the central region confirms the repercussions of dysversion using both perim-etry testing and OCT.

The macular ganglion cell complex is simpler to evaluate in dysversion as it is located at a distance from the optic nerve. It allows revealing the normality or the existence of optical fibre impairment.

132

Bilateral physiological excava-tion. The RNFL is normal. The optic nerve parameters are in grey (not statistically analysed), as the de-vice does not have such large disc surfaces in its data base.

Right eye

Left eye

Physiologicalexcavation

Using OCT, the large papilla, which are responsible for physiological excava-tions, are expressed as a preservation of the RNFL and the macular ganglion cell complex. The optic nerve parameters are not often compared with a nor-mative value because the data bases do not in general include physiological excavation.

133

Same patient as on the previous page. The ganglion cell complex is normal. Bilateral physiological excavation.

Physiological excavation

In the case of important and possibly physiological excavation, the optical fibre parameters should be analysed and those of the optic nerve should be considered to a lesser degree.

134

Nasal coloboma of the right papilla. OCT impairment is oppo-site the abnormal region.

Right eye

Left eye

Papillarycoloboma

Localisation of the coloboma is responsible for the opposite OCT. The sectors that are usually affected in glaucoma, the lower temporal region in particular, are not affected if this region is the seat of the coloboma.

135

Nasal coloboma of the right papilla. Same patient as on the previous page. The ganglion cell complex is normal.

Papillary coloboma

Certain macular or retinal pathologies can alter the optical fibre layer, at the macular level, but also at the peripapillary level. Therefore, a change in these regions does not necessarily indicate direct impairment of the optic nerve. If initial retinal impairment is mainly macular, we frequently only observe a change in the macular ganglion cell complex. If impairment is more impor-tant, all the structures can be altered. Two examples are shown here, but they are not exhaustive as regards retinal pathologies possibly suggesting optic neuropathy.

AtypicalaspectsoftheretinapossiblysuggestingopticalneuropathyusingOCT

136 137

138

72-year-old male patient affected 3 years earlier by upper temporal branch occlusion in the right eye. Both OCT and visual field testing detect the corresponding impair-ment.

Veinocclusionsequelae

Vein occlusion is responsible for the destruction of the ganglion cell layer in the same way as impairment of the optic nerve. The context allows an under-standing of the OCT aspect.

139

Same patient as on the previous page. OCT and central 10° visual fields. We observe diffuse and uni-lateral impairment of the macular ganglion cell complex.

Vein occlusion sequelae

140

23-year-old male patient with pigmentary retinopathy. Gold-mann’s visual field testing shows important bilateral peracentral im-pairment. The peripapillary optical fibre layer remains within normal limits.

Right eye

Left eye

Pigmentaryretinopathy

Impairment of the macular ganglion cell complex is very frequent in retinal pathologies. It is even encountered when initial impairment is not located in the optical fibre layer, such as pigmentary retinopathy.

141

Same patient as on the previous page. We observe diffuse and bi-lateral impairment of the macular ganglion cell complex.

Pigmentary retinopathy

The example given here corresponds to pigmentary retinopathy. All the other sources of macular injury can lead to impairment of the macular ganglion cell complex: epiretinal membrane, DMLA, macular oedema of any origin, macu-lopathies acquired or constitutional…

143

Conclusion

Optic nerve imaging using OCT is developing constantly. Most of the refer-ences of this publication are recent because earlier ones do not concern the “Spectral Domain”. In the last year, more than 300 articles on “Optic nerve and OCT” have been published in international journals, underlining the inno-vative character of this technology.

Many questions arise in view of these spectacular results: should OCT be con-sidered a fundamental examination in the context of glaucoma monitoring or is it just supplementary to visual field testing and ocular pressure measure-ment? Can we ignore visual field testing, a limiting examination that is not well accepted by a large number of patients? What should one do if the OCT worsens significantly without an alteration in the visual field? On the contrary, what should one think of a visual field that is degrading without confirmation of change using OCT?

At the moment, it is reasonable to consider on the one hand that OCT has become a key examination for glaucoma and on the other hand that it cannot replace visual field testing.

The same type of questions are posed for other optic neuropathies. Should OCT eventually replace visual field testing? What should one think of OCT impairment in the absence of clinical optic nerve impairment?

OCT is still developing in terms of image precision and quality. New struc-tures are being studied and measured, such as the cribriform plate. Other developments are possible, in particular the capacity to analyse not only layer thickness but also layer content. This would be very useful for counting e.g. the number of residual ganglion cells in optic nerve diseases without integrat-ing the support cells that are responsible for the persistence of this layer, even after total destruction of the visual fibres.

The use of large wavelength laser beams (1000 nm or more, such as SWEPT-OCT) also allows studying structures beyond the pigmentary epithelium of the retina, but at the cost of lower image quality. The choroid and the region beyond the cribriform plate of the papilla also become visible. In the future, these technologies will allow an understanding of certain complex pathologies involving the modification of peripapillary cladding structures and cribriform plate impairment.

OCT has thus become a basic examination for the evaluation of optic neu-ropathies, without however totally replacing visual function analysis, and automated perimetry testing in particular.

145

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